In recent years, contact-free human-machine interfaces (cfHMIs) have been developed rapidly. According to the research paper authored by an analyst from Forrester Research, Inc., as long as the motion sensing technology that revolutionizes human beings' interactions with electronic devices has fully crept into our daily lives, it will present us a vision of future interactive experiences. At present, a number of manufacturers have been dedicated to creating various products that may be applied in our daily lives. For instance, Kinect, the new era motion sensing input device launched by Microsoft, enables gamers to interact with the console by means of human gestures and movements without the need to touch a game controller; at the exhibition hall of Boeing, an interactive virtual simulator allows people to experience 3D flight simulation.
Depth images provide complete spatial image information, and therefore how to effectively, timely, and stably obtain the information of the third dimension (i.e., the depth) is essential to the development of interactive virtual input technologies. According to the related art, a depth map technology that may achieve “spatial 3D interaction” has drawn most of the attention, whereas the absolute coordinate of hand motion and even delicate finger motion can be barely obtained through the depth map estimation due to the existing limitations of distance, resolution, and so on. As such, it is rather difficult to apply the depth map technology to meet small-range cfHMI requirements.
The existing interactive input devices are mostly applied for human-machine interactions within a wide range, such as a large-sized immersive interactive virtual device, an interactive digital blackboard, a motion-sensing interactive game, and so forth, whereas an object moving within a rather small range in the 3D space is not apt to be accurately positioned. For instance, the existing interactive input technology is not suitable for being applied to capture fine and short-ranged motions of hand-sized objects. Although some interactive devices equipped with handheld infrared projectors or markers may track and recognize users' gestures or motions, these interactive devices are merely applicable in case of the wide interaction range. Subject to the fixed projection image area and the imprecise dimensions of markers, the conventional interactive technologies are not yet applicable to a portable, contact-free 3D human-machine interactive interface that may capture short-range motions.